Download Big Bottle System User`s Manual

BIG BOTTLE RAD H2O
High Sensitivity Radon in Water Accessory
User Manual
Revision 2015-02-16
DURRIDGE Company Inc.
524 Boston Road
Billerica, MA 01821
Tel: (978) 667-9556
Fax: (978) 667-9557
[email protected]
www.durridge.com
INTRODUCTION
The BIG BOTTLE RAD H2O is an accessory to the RAD7 radon monitor that enables you to measure radon in
water over a concentration range of from 1 pCi/L to greater than 10,000 pCi/L. For higher ranges, the standard
RAD H2O accessory is recommended.
The complete system, including the RAD7, is portable and battery operated, and the measurement is moderately
fast. Even for the lowest radon concentrations you can have an accurate reading of radon in the water within
two hours of taking the sample. The BIG BOTTLE RAD H2O gives results after 120 minutes analysis, or less,
with a sensitivity that far exceeds that of liquid scintillation methods. The method is simple and straightforward,
though more complicated than with the standard RAD H2O. There are no harmful chemicals to use. Once the
procedure becomes familiar and well understood it will produce accurate results with minimal effort.
It is assumed that the user has a good, working knowledge of the RAD7. If both the RAD7 and the BIG
BOTTLE RAD H2O are new to the user, then time should be spent learning how to make good measurements of
radon in air with the RAD7 before embarking on radon in water measurements. Instructions for RAD7
operation specifically with the BIG BOTTLE RAD H2O are given in this manual but, for more detail about the
instrument and its use to measure radon and thoron in air, the reader is referred to the RAD7 manual.
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© 2015, DURRIDGE Company Inc.
TABLE OF CONTENTS
1 GETTING STARTED!
5
1.1 Unpacking!
5
1.2 General Safety Instructions!
5
1.3 Taking a Look!
5
1.4 Running a Test!
8
1.4.1 Preparing the RAD7!
1.4.2 Collecting a Sample!
1.4.3 Setting up the equipment!
1.4.4 Running the test with desiccant!
1.4.5 Running the test with the DRYSTIK!
1.4.6 Finishing the Test!
1.4.7 Interpreting the results!
2 RAD H2O MEASUREMENT TECHNIQUE!
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13
2.1 The Closed Loop Concept!
13
2.2 Desiccant!
13
2.3 Purging the System!
13
2.4 Background and Residuals!
14
3 CALCULATING RESULTS!
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3.1 How Calculation Is Made!
16
3.2 Detailed Calculation!
16
3.2.1 Definitions!
3.2.2 Known Values!
3.2.3 Head Space Loss!
3.2.4 Air-equivalent of water!
3.2.5 Calculation!
3.2.6 Correction for ambient/purge radon!
3.2.7 Radon Graph Interpretation!
3.2.8 Decay Correction!
3.3 Automatic Calculation with CAPTURE Software!
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4 ACCURACY AND QUALITY CONTROL!
4.1 Calibration of System!
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4.2.1 Sampling Technique!
4.2.2 Sample Concentration!
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© 2015, DURRIDGE Company Inc.
4.2.3 Sample Size!
4.2.4 Counting Time!
4.2.5 Temperature!
4.2.6 Relative Humidity !
4.2.7 Background Effects!
4.3 Comparison of RAD H2O and BIG BOTTLE with Other Methods!
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4.4 Quality Assurance!
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5 CARE, MAINTENANCE and TROUBLE SHOOTING!
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5.1 Warning on Pump Direction!
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5.2 Warning on Tipping the Aeration Unit!
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5.3 Stones Maintenance!
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5.4 High Humidity !
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5.5 Foaming!
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5.6 Glass Jug Care!
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5.7 Technical Support!
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6 BACKGROUND TO RADON-IN-WATER MEASUREMENT!
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6.1 About Radon-in-Water!
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6.2 Health Risks Due to Waterborne Radon!
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6.3 Physical Properties of Waterborne Radon!
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6.4 Radon as a Tracer for Groundwater movement!
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6.5 Standard Methods for Radon in Water Analysis!
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6.6 Mitigation Strategies!
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REFERENCES!
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© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
1 GETTING STARTED
1.1 Unpacking
1.2 General Safety Instructions
First make sure you have everything you are
supposed to have. Examine the case contents and
verify that you have all the items shown in the
packing list. If anything is missing, please call
DURRIDGE immediately at (978) 667-9556 or email
[email protected].
There is nothing hazardous to the user in the BIG
BOTTLE RAD H2O, but care should be taken to
make sure that water never enters the RAD7. The
check valve attached to the aerator should never be
removed, as it protects the RAD7 in the event that
the tube connections to the instrument are reversed.
1.3 Taking a Look
Fig. 1 The BIG BOTTLE RAD H2O Glass Jug Kit (left) and Soda Bottle Kit (right)
(Note: the plastic soda bottle is not included.)
The BIG BOTTLE RAD H2O system consists of
several components. Depending on whether the
system is purchased for use with samples collected in
a glass jug or a soda bottle, it will come with either
the Glass Jug Kit (see Fig. 1, left) or the Soda Bottle
Kit (see Fig. 1, right.)
The Glass Jug Kit comes with two 2.5 L bottles and
an aeration system, while the Soda Bottle Kit comes
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with only an aeration system. It is compatible with
most 500ml - 2L pressurized PET soda bottles, sold
separately.
If the Soda Bottle Kit is being used, be aware that
large soda bottle with insufficient wall strength may
be become dented inward during aeration. Any
indentation will reduce the volume of the bottle and
displace water upward towards the bubble trap and
© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
desiccant. For this reason bottles with strong walls
that can resist denting are recommended. This
problem is less likely to affect soda bottles with
volumes of less than 2L. Glass bottles are not
affected.
Each aeration system includes a check valve and one
or two aeration stones. Both the Glass Jug and the
Soda Bottle systems include the Temperature Logger
Kit and the Retort Stand Kit, which are detailed on
the following page. The Temperature Logger Kit
(see Fig. 2, left) consists of a Lascar Temperature
probe with a USB cable and necessary software, a
hermetically sealed thermocouple, and an elastic,
padded strap for attaching the thermocouple to the
bottle.The Retort Stand Kit (see Fig. 2, right)
consists of a short stand, clamp, Bubble Trap, and
tubing set with inlet filter.
Any BIG BOTTLE system will require a RAD7
radon monitor, plus either a Laboratory Dryer or
DRYSTIK. These items must be purchased
separately.
An example of a fully assembled BIG BOTTLE
RAD H2O system is shown in Fig. 3 on the following
page.
Fig. 2 Temperature Logger Kit (left) and the Retort Stand Kit (right)
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© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
Fig. 3 BIG BOTTLE RAD H2O Schematic
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© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
1.4 Running a Test
1.4.1 Preparing the RAD7
Before making a measurement, the RAD7 must be
free of radon and dry. To achieve this, it should be
purged for some time. A number of drying options
may be chosen for the measurement itself, but it is
convenient to use the large laboratory drying unit
during the purging process, to save the small drying
tubes for use with the actual measurement.
Hook up the laboratory drying unit to the RAD7
inlet, with the inlet filter in place (see RAD7
manual). Any tube connected to the outlet must be
open to the air at the other end. Purge the unit with
fresh dry air for ten minutes.
After 10 minutes of purging with dry air, push the
[MENU] button, then push [ENTER] twice, to go to the
status window, and push the right arrow button twice
to see the relative humidity. If it is not yet down
close to 10%, start purging some more. To conserve
desiccant, after the first ten minutes or so of purging,
you may connect the RAD7 outlet to the inlet of the
laboratory drying unit, thus forming a closed loop.
This will continue to dry out the RAD7 but will no
longer purge the unit with fresh air.
If the RAD7 has not been used for some time, or if it
has been left without the small tubing bridge in place
between the air inlet and outlet, then it may take an
hour or more to dry it out. Once it has thoroughly
dried out, however, just 15 minutes of purging and
drying between measurements of different samples
will generally be sufficient.
1.4.2 Collecting a Sample
Getting a good sample requires care and practice.
Sampling technique, or lack of it, is generally the
major source of error in measuring the radon content
of water. The water sampled must be a)
representative of the water being tested, and b)
delivered to the BIG BOTTLE without ever being in
contact with air.
To satisfy (b), one method is to attach a tube to a
faucet, and insert the tube into the bottle so that it
reaches the bottom. Fill the bottle and allow water to
overflow for a while, to ensure that it is filled with
water that has not lost any radon to the surrounding
air. After filling a 2.5L glass jug, immediately decant
a few ml so that the water level is down near the
flange below the thread on the neck of the bottle.
Then replace the cap. The air-space in the bottle
provides an expansion buffer and prevents the bottle
from being broken by expansion of the water as it
warms up to room temperature.
Another method, suitable for sampling in open water,
is to lower the empty bottle upside down to the depth
required and then inverting the bottle so that the air
inside is displaced by water. This method has a
disadvantage in that the air in the bottle, when it
leaves, will take some of the radon out of the water
as it replaces the air in the bottle, so that the sample
may be deficient in radon. A better solution is a
combination of both methods, in that a pump may be
used to flush water into the bottle while it is
submerged, thus replacing the deficient water with
fresh water sampled from the same depth. A cap
may be placed on the bottle while it is being
retrieved, but for a 2.5L glass jug, this should
immediately be removed and a few ml of water
decanted off to create the expansion buffer.
Dry the bottle and apply a label stating the date, time
and source of the water.
1.4.3 Setting up the equipment
The method described in this section uses the large
laboratory drying unit. Other choices are discussed
in Section 3.
The following should be attached to the aerator:
•
•
•
•
Teflon collar
Bottle cap
Tubing with Y-Connector
Aerator Stones
To satisfy (a), make sure that the sample has not been
through a charcoal filter, or been sitting for days in a
hot water tank. To test a well, choose a faucet at the
well, or outside the house, before the water enters
any treatment process. Run the water for several
minutes, to make sure that the sample comes fresh
from deep in the well.
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© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
aerator. Connect a bypass from one T-connector to
the other, with a shut-off clip on the tubing. See Fig.
3.
With the system as connected so far, set the RAD7 to
purge for another few minutes. While
it is purging, clamp the drying unit to the retort stand,
thus supporting it about 70cm above the base, such
that the cap attached to the aerator is at the right
height to fit on the BIG BOTTLE.
Press [Enter] on the RAD7 to stop purging. Then
select [Setup]→[Protocol] →[Sniff] and
push [Enter]. Next go to [Setup]→[Cycle]
and set the cycle time to 15 minutes. Set the Format
to short by choosing [Setup]→[Format]
→[Short] and pushing [Enter]. Once these
settings have been configured, it may be desirable to
save the configuration under the User Protocol. This
will make it easier to restore the necessary settings in
the future. (This is described in more detail in the
RAD7 manual.) Make sure the infrared printer has
paper, and switch it on. Next switch off the RAD7,
then switch it on again. It will print its identity and a
review of the setup.
Fig. 4 Aerator and Bottle assembly with
Temperature Logger
The temperature logger, with the waterproof sensor,
may be started logging the temperature any time
prior to the water measurement. See the temperature
logger manual. Choose a 2-minute time interval
between temperature readings.
Assemble with 2” (5cm) of large, 5/16” ID vinyl
tubing connecting the output of the aerator barrel to
the bubble trap. Connect the other end of the bubble
trap to a T-connector and thence to the screw-cap end
of the large drying unit, also with 5/16” ID tubing.
The other end of the large drying unit is connected,
with 1/8” ID tubing, to an inlet filter mounted on the
RAD7 inlet. Adaptors from 5/16” to 1/8” makes this
connection easy and secure. Connect the RAD7
outlet (also with 1/8" ID tubing, adapted up to 3/16")
to a T-connector and thence to the check valve on the
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While the RAD7 is printing, remove the cap from the
BIG BOTTLE and lower the aerator stones into the
water. Some water may spill during this procedure.
Screw the cap attached to the aerator onto the BIG
BOTTLE. The bottle should stand comfortably on
the base of the retort stand with the bubble trap and
desiccant supported vertically above it. Attach the
temperature sensor in direct contact with the glass of
the bottle and held in place with the foam under the
elastic clinching strap. Close the clip on the bypass.
1.4.4 Running the test with desiccant
Once everything is set up, make sure the
temperature logger is running. Once the RAD7 has
finished printing out the header, go to Test→Start
and push [ENTER]. The pump will start running,
aerating the sample and delivering radon to the
RAD7. Set the pump to run continuously by
selecting [Setup]→[Pump] →[On] and pushing
[Enter]. After 15 minutes, the RAD7 will print out
a short-form report. The same thing will happen
again 15 minutes later, and every 15 minutes after
that.
© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
Before starting the test, turn the RAD7 pump off
([Setup]→[Pump] →[Off] →[Enter]).
Connect the low-flow outlet of the DRYSTIK to the
inlet of the RAD7 and the outlet of the RAD7 to the
purge input on the DRYSTIK. Run the DRYSTIK at
100% duty cycle for an hour or more, to bring down
the RH in the RAD7 and to purge it with fresh air.
Connect the system as shown in Fig. 6. What was
previously (with desiccant) connected as a bypass to
the aerator is now used to make the high flow output
from the DRYSTIK available for rapid aeration of
the sample. Once everything is set up, the
DRYSTIK may be started with 100% duty cycle.
This starts the aeration of the sample and the
distribution of radon around the air loop. The RAD7
(with the RAD7 pump still off) should also be
started. Every 15 minutes it will complete a cycle
and print out the data if a printer is in position and
switched on.
Fig. 5 Aeration in progress
Aerating the water in the BIG BOTTLE RAD H2O
moves the bulk of the radon from the water into the
air loop. This process may take around 45 minutes,
depending on several factors.
After about 45 minutes from the start, the aeration
process will be complete and the radon in the air loop
will be close to equilibrium with the radon remaining
in the water. The clip between the T-connectors may
now be opened to allow the air flow to bypass the
bottle.
After 45 minutes or so (with the higher flow rate) the
aeration will be complete. At this point, the clip on
the bypass may be closed, thus shutting off the flow
from the high-flow output (it may be necessary to put
a tie-wrap around the upstream hose connections to
prevent them blowing off if they are not tight). Also
the duty cycle can be reduced to, say, 20% thus
reducing the air flow round the loop to a sedate 0.03
L/min, which is sufficient to keep the RAD7 dry.
Keep the measurement going until sufficient data
points have been accumulated to reach the precision
required.
At this point, it is no longer necessary to keep the
pump running continuously. The pump may be set to
Auto by selecting [Setup]→[Pump] →[Auto]
and pushing [Enter]. To read the relative humidity
go to the third status window, see section 1.4.1.
1.4.5 Running the test with the DRYSTIK
The 48-inch and 144-inch DRYSTIKs devices are
both capable of bringing down the RH in the RAD7
to below 10 without the use of desiccant. The 12inch DRYSTIK is not quite able to do that, but brings
the RH down below 20, which gives good results
when CAPTURE is used to make the automatic
correction for humidity. Alternatively, the 12-ADS-2
may be used with a small drying tube placed between
the ADS-2 and the RAD7 inlet, and will extend the
life of the drying tube significantly.
Fig. 6 BIG BOTTLE with DRYSTIK
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© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
1.4.6 Finishing the Test
If left alone, the measurement will continue
indefinitely. At the end of each 15-minute cycle the
RAD7 will print out the measurement of radon in air
and its uncertainty. During the first 45 minutes the
RAD7 will aerate the sample and respond to the
growth of radon in the measurement chamber, so the
cycles that are completed during this time should be
ignored.
The best time to stop the measurement will depend
on the level of certainty required, but it will always
occur after the initial 45 minute aeration has been
completed.
Radon concentrations and uncertainties are shown in
the top line of the printout for each cycle. You can
also look at the last reading in the second status
window, by pressing [Menu] then [Enter],
pressing[Enter] again and the right arrow once.
Observe the uncertainty and express it as a
percentage of the base value. If that percentage
uncertainty is too high, calculate the ratio of this
observed uncertainty to an acceptable uncertainty.
The square of that ratio is the number of cycles, N,
after equilibrium, that are required to reach the
desired precision of measurement.
You can, if you wish, set the RAD7 to terminate the
measurement at the required point automatically by
setting the “Recycle” number to ‘N + 2’:
[Setup]→[Recycle] →[‘N+2’],[Enter].
At the end of the (N+2)th cycle the RAD7 will finish
the measurement and print out a run summary.
Alternatively, once the (N+2)th cycle has been
completed and the data printout finished, you can
simply turn off the RAD7. This will terminate the
measurement. The data will be stored in the RAD7
and on the printer output, but there will be no run
summary printed out.
If no more tests are to be analyzed, the equipment
may now be dismantled. If there is another sample
for analysis, keep the RAD7 connected as above
(with the water jug removed) and purge for ten
minutes. Check the relative humidity, as above, and
continue the purge until the relative humidity
indication in the instrument drops to 10% or below.
After ten minutes, the RAD7 air outlet may be
connected to the input of the drying unit by opening
the bypass clip, to form a closed loop, to conserve
desiccant. When the relative humidity is down to
10% or less, another test may be conducted. Repeat
from 1.4.1 above.
1.4.7 Interpreting the results
The RAD7 printout may appear something like Fig.
7, if the recycle number was set to N+2. Otherwise,
if the measurement was terminated by switching off
the RAD7, the printout will consist only of the cycles
data. The run summary will be missing.
Download the RAD7 data to your computer with
CAPTURE. Set the Radon Measurement Method to
Big Bottle RAD H2O for the automatic calculation of
radon in water concentrations based on the RAD7’s
reported radon in air values, or set the Radon
Measurement Method to Radon in Air if these values
are to be calculated manually. See Section 3 for
details.
In addition, there will be a temperature log stored in
the Lascar temperature logger, that may be
downloaded and printed with a Windows PC. Use
the PC software to create a comma-delimited ASCII
text file of the temperature data. For the automatic
conversion of the RAD7 radon-in-air readings to
radon concentration in the water sample, CAPTURE
will need that temperature data.
The final step is to correct the measured value of
radon in the water for decay of the radon during the
time between taking the sample and analyzing it.
See Section 3 for details.
Once the measurement is terminated, unscrew the
bottle cap, raise the aerator stones out of the water,
close the bypass clip and set the RAD7 to purge for 8
minutes or more. This will blow water out of the
stones, and also introduce fresh air into the return
tubing and the RAD7.
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© 2015, DURRIDGE Company Inc.
1 GETTING STARTED
Fig. 7 RAD H2O printout
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© 2015, DURRIDGE Company Inc.
2 RAD H2O MEASUREMENT TECHNIQUE
2 RAD H2O MEASUREMENT TECHNIQUE
2.1 The Closed Loop Concept
The RAD H2O method employs a closed loop
aeration scheme whereby the air volume and water
volume are constant and independent of the flow
rate. The air recirculates through the water and
extracts the radon until a state of equilibrium
develops. The standard RAD H2O system reaches
this state of equilibrium within 5 minutes, after
which no more radon can be extracted from the
water. The BIG BOTTLE RAD H2O needs aeration
for more than 10 minutes to reach equilibrium.
With a standard RAD H2O, the extraction efficiency
of radon from the water samples is very high, and
temperature independent, typically 99% for a 40 mL
sample and 94% for a 250 mL sample. For the BIG
BOTTLE RAD H2O, however, the extraction
efficiency is temperature dependent and the
temperature has to be monitored.
2.2 Desiccant
The BIG BOTTLE RAD H2O is more efficient if
desiccant is used at all times to dry the air stream
before it enters the RAD7. A DRYSTIK model 48ADS-2 or 144-ADS-2 will keep the RAD7
reasonably dry, even without any desiccant. If
desiccant or DRYSTIK is not used, the RAD7 will be
filled with humid air and the readings will need to
be corrected for humidity to obtain correct radon
concentrations. This can be done automatically by
CAPTURE, which may be downloaded from the
DURRIDGE web site, (http://durridge.com).
The BIG BOTTLE RAD H2O system may be used
with any of; the large drying unit, the small drying
tube, a DRYSTIK or none. Of these, the best
solution is probably the DRYSTIK 144-DAS-2 as
this keeps the RAD7 dry while minimizing the
volume of the air loop.
2.3 Purging the System
After performing a water or air measurement, the
RAD7's internal measurement chamber will continue
to contain the radon that was measured. If this radon
is still present in a closed air loop method, such as
the BIG BOTTLE RAD H2O, when you start a new
measurement the retained radon in the chamber will
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be added to the radon stripped from the water. To
prepare for the next water measurement, therefore,
you must remove the old radon from the RAD7 and
its peripherals, including the aerator heads, tubes,
and desiccant. This procedure is known as "purging
the system."
To purge the system, you must have a source of
radon-free (or relatively radon-free) air or inert gas.
For most occasions ambient air is good enough, but
see below for a discussion on background. Put the
RAD7 into a purge cycle with the
[Test]→[Purge] command, and allow the RAD7
pump to flush clean air through the entire system for
10 minutes. Do not recycle the purge air back
through the desiccant to the RAD7 until after at least
10 minutes of open-loop purging to clear out the
radon.
Be sure to allow all the hoses and fittings to flush
thoroughly by keeping them attached during the
purge cycle for at least the first five minutes. Also be
sure that the drying tube does not deplete its
desiccant during the purge cycle. If the depleted
(pink) desiccant gets to within 1 inch of the drying
tube outlet, replace the tube with a fresh (blue)
drying tube. After the first two or three minutes of
purging, you may replace a small drying tube with
the large laboratory drying unit, to conserve the small
drying tube desiccant, and continue purging the
system.
Be careful about the air you use to purge! Ambient
air may not be adequately free of radon to properly
prepare the system for a low level sample. The radon
present in the purge air will add unwanted
"background" to the next measurement. For
example, a purge air radon concentration of 4 pCi/L
will add about 5 pCi/L (depending on the BIG
BOTTLE configuration) additional radon
concentration to the next water result. This would be
too much background to neglect when measuring
samples below 20 pCi/L, but if you are measuring
only water samples above 20 pCi/L, you may
consider this amount of offset to be acceptable. To
reduce the error due to purge air radon you may
either subtract off the background from every
measurement, or adopt strategies, such as use of
activated charcoal during the purge, or purging with
nitrogen gas or outside air, to reduce the background
to acceptable levels.
© 2015, DURRIDGE Company Inc.
2 RAD H2O MEASUREMENT TECHNIQUE
One way to determine the background is to measure
a "blank", a water sample containing no radon. To
get radon-free water, purchase distilled water from
your local pharmacy, or fill a container with tap
water, and allow the container to stand closed and
undisturbed for 4 weeks or more. The 4 week period
allows any radon present in the water to decay away.
Store your radon free water in a closed air-tight
container. Remember that the background due to
purge air radon will change when the air radon
concentration changes, so if you intend to subtract
background you should measure a blank sample at
every measurement session.
An alternative method to determine background is to
make a measurement of the air in SNIFF mode and
note the equilibrium count rate in window A.
After purging the RAD7 with fresh air, start a
measurement. After 15 minutes start a new cycle.
After another ten minutes look at the count rate in
Window A, in the fifth status window. From a
previous printout of a water measurement, with the
format set to medium or long, you can see the count
rate in window A corresponding to the water radon
concentration measured. Typically, for a 2.5L BIG
BOTTLE, 100 pCi/L in the water will generate about
20 cpm in window A. A background count rate of
0.5 cpm in window A (equivalent to about 2 pCi/L in
air) will then produce an offset of 2.5% in the final
reading.
The obvious way to reduce background is to purge
with very low radon air. Outdoor air rarely exceeds
0.5 pCi/L at several feet above the ground, so you
can probably get the water background to below 0.7
pCi/L by simply using outdoor air to purge. To get
even lower radon air, fill a tank or balloon with
outdoor air and let it age for several weeks. If you
are using compressed air or inert gas, be very careful
not to allow the RAD7 to pressurize, as this may
cause internal damage to the pump or seals.
Another method to reduce background is to use
charcoal adsorption to clean the remaining radon
from the system following the purge. A small tube
containing 15 grams of activated charcoal, as
supplied with the RAD H2O, can remove up to 98%
of the remaining radon from the RAD7 when
connected in a closed loop. This will reduce the
system's radon to truly negligible levels for the most
accurate low level radon-in-water measurement. The
charcoal filter works best if you use it only after a
complete purge with low radon air, which avoids
overloading the filter with radon. If the charcoal
filter becomes badly contaminated it can give off
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some radon and actually increase the background
after a purge. Store the charcoal filter with the
yellow end caps installed to allow the filter to "selfclean" by waiting for adsorbed radon to decay over
several weeks time. Always keep the charcoal dry by
using it in conjunction with a drying tube, since
water vapor can adversely affect charcoal's capacity
to adsorb radon.
Even if you choose not to use elaborate methods to
reduce the background, you should always purge the
system between samples. It is much better to purge
with ordinary room air than not to purge at all.
2.4 Background and Residuals
Purge air is one among several causes for
background counts in the BIG BOTTLE RAD H2O.
The most significant other causes are radon
daughters and traces of radon left from previous
measurements. The RAD7 has the unusual ability to
tell the difference between the "new" radon
daughters and the "old" radon daughters left from
previous tests. Even so, a very high radon sample
can cause daughter activity that can affect the next
measurement.
After a high radon sample has been purged from the
system, its decay products stay behind until they
decay away. The polonium-218 isotope decays with
a 3 minute half-life. In the 30 minutes following the
purge, the polonium-218 decays to about a
thousandth of its original activity. That still leaves a
background of 100 pCi/L after a 100,000 pCi/L
sample.
In addition to polonium-218, the RAD7 is sensitive
to polonium-214, which can give counts for several
hours after the radon has been removed. The RAD7
uses alpha energy discrimination to reject
polonium-214 counts from a measurement, but a
very small percentage of the polonium-214 decays
slip past the discriminator. This can add background
to a measurement that follows a high radon sample.
The solution to the problem of daughter activity is
time. Simply wait for the activity to decay away.
Check the background with a blank sample. If it is
still too high, keep waiting, and keep checking. The
length of time you will wait depends on just how
much radon your high radon sample had, and just
how much background you are willing to tolerate
before the next measurement. If you expect the next
sample to be high also, you may want to go ahead
with the next measurement right away, considering a
small amount of background acceptable.
© 2015, DURRIDGE Company Inc.
2 RAD H2O MEASUREMENT TECHNIQUE
In the case of extremely high radon samples, you
may develop a background that is more persistent
than daughter activity. That is possibly due to offgassing of residual radon that has absorbed into
internal surfaces. In particular, rubber and plastic
parts can absorb a small fraction of the radon that
passes through the system. A small fraction of a very
large amount can still be significant. The radon may
desorb from these materials over many hours. In the
worst case you may have to allow the system to sit
idle for a day or more for the absorbed radon to
finish leaking out of these materials, then purge the
system again to remove the radon. A radon
15
concentration high enough to cause a concern of this
kind is very rare in natural ground water, but is
possible in artificial radon sources such as radium
crocks or "Revigators".
Sustained counting of very high radon concentrations
can lead to the buildup of long lived lead-210
contamination of the RAD7's alpha detector. This
possibility is described in the RAD7 Operator's
Manual. It suffices to say that the RAD7's ability to
distinguish alpha particles by energy makes it far less
susceptible to the build up of lead-210 related
background than other radon monitors.
© 2015, DURRIDGE Company Inc.
3 CALCULATING RESULTS
3 CALCULATING RESULTS
3.1 How Calculation Is Made
For the standard RAD H2O, the RAD7 uses the WAT
40 and WAT250 protocols to automate the aeration
and counting process and to calculate the radon
concentration in the water directly, so that the RAD7
reading is the actual radon concentration in the water.
With the BIG BOTTLE RAD H2O system, however,
the WAT protocols should generally be avoided.
Instead, the RAD7 is used in Sniff mode to measure
the radon concentration in the air loop and a
conversion factor to radon in the water must be
applied. The conversion factor used depends on the
configuration of the BIG BOTTLE system and on the
temperature of the water at the time of the aeration.
Automatic conversion is not performed by the RAD7
as in RAD H2O experiments, but is instead handled
using CAPTURE. The physical configuration of the
experiment needs to be specified, specifically the size
of the bottle and tubing apparatus, the type of drying
unit used, and the temperature of the water at the time
of aeration. Without CAPTURE, this calculation
may be performed manually as shown below.
3.2 Detailed Calculation
The basic principle is that the radon, originally in the
water, is distributed around the air loop. The
concentration in the air loop is measured and from
this the original radon concentration in the water is
calculated.
Rn
total radon in system
∆ Rn radon lost from system
Cw
original radon concentration in the water
Cb
background radon concentration in air
Ca
equilibrium radon concentration in air
T
temperature of water in bottle
∝
equilibrium coefficient from Fritz von Weigel
equation.
3.2.2 Known Values
VR7
Vd
Vt
Vb
Vh
Vw
768ml, standard RAD7
963ml, RAD7 with oversized dome
400ml, laboratory drying unit
15ml, small drying tube
20ml, Active DRYSTIK
54ml
51ml
15ml
2.5L
It may be noted that
• 5/16” ID tubing has a volume of 49ml/m.
• 3/16” ID is 18ml/m.
• ⅛” ID is 8ml/m.
The standard tubing set results in a total tubing
volume, Vt, of 54ml.
Va = VR7 + Vd + Vt + Vb + Vh
Assuming that the set up is standard,
Vt + Vb + Vh = 120ml. Giving:
3.2.1 Definitions
Va = 120 + VR7 + Vd
To begin, we need to identify all the variables
involved. There are several volumes, designated with
a V and a lower case suffix, total radon, Rn, radon
concentrations, C, and temperature, T.
With a standard RAD7 and a laboratory drying unit:
3.2.3 Head Space Loss
Let:
V
VR7
Vd
Vt
Vb
Vh
Va
Vw
Veq
Va = 120 + 768 + 400ml = 1.29L
total equivalent air volume of system
internal volume of the RAD7
air space & pore volume of desiccant
volume of tubing & aerator
volume of bubble trap
volume of head space in bottle
total volume of air in the system
volume of water in bottle
equivalent air volume of water in bottle
16
The head space of air above the water in the BIG
BOTTLE is there to prevent expansion of the water
from breaking the bottle. However it causes a loss of
radon from the sample in that the radon concentration
in the head-space air will reach equilibrium with the
water and, when the cap is opened, that air will
escape.
If we assume a typical value for ∝, the equilibrium
coefficient, at typical room temperature, to be 0.25,
© 2015, DURRIDGE Company Inc.
3 CALCULATING RESULTS
then the radon concentration in the head space will be
four times the radon concentration in the water.
Radon lost from the system will be:
∆ Rn = (Cw ⋅ Vh) / ∝
Care needs to be taken to ensure that the units for
concentration and volume are consistent, e.g. if Cw is
in Bq/m3 then Vh must be expressed in m3.
3.2.6 Correction for ambient/purge radon
Any radon in the purging air will simply be added to
the radon in the system. Therefore:
Rn = Cw ⋅ (Vw - Vh /∝) + Cb ⋅ Va
This modifies the equation, above, to:
Cw = [Ca ⋅ (Va + ∝Vw) - Cb ⋅ Va] / (Vw - Vh /∝)
3.2.4 Air-equivalent of water
When the radon is distributed uniformly around the
air loop, some will remain in the water. The
equilibrium ratio of the concentrations is ∝, as
determined by the von Weigel equation. A volume
Vw of water will therefore contain as much radon as a
volume ∝ Vw of air. This quantity, ∝Vw, may
therefore be considered the air-equivalent volume of
the water.
3.2.5 Calculation
Rn = Cw ⋅ Vw - ∆ Rn
Rn = Cw ⋅ (Vw - Vh / ∝)
The total equivalent air volume of the system will be
Va + ∝Vw. The total radon in the system, Rn, will be
distributed around this volume, so that the
concentration in the air loop at equilibrium will be:
which is nearly the same as:
Cw = (Ca - Cb) ⋅ (Va + ∝Vw) / (Vw - Vh /∝)
3.2.7 Radon Graph Interpretation
The graph of radon against time generated by
CAPTURE from the RAD7 data is not a horizontal
straight line. There is, first, a rising slope due both to
the response time of the RAD7 in Sniff mode and to
the time taken to transfer radon from the water to the
air. The curve goes through a small peak and then
settles on a slowly descending straight line.
substituting for Rn:
The intercept of this line on the Y axis, projected
back to the start of the measurement, would be a
good reading. However, it is not necessary to keep
the measurement going long enough to determine the
position and slope of that line with good accuracy. It
is sufficient to average the readings from 45 minutes
to between 60 and 90 minutes from the start of the
measurement, depending on the radon level and the
precision required.
Cw ⋅ (Vw - Vh /∝) = Ca ⋅ (Va + ∝ Vw)
3.2.8 Decay Correction
or
If you collect a sample and analyze it at a later time
(rather than immediately), the radon concentration
will decline due to the radioactive decay. You must
correct the result for the sample's decay from the time
the sample was drawn to the time it was counted. If
the sample is properly sealed and stored, and counted
within 24 hours, then the decay corrected result
should be almost as accurate as that of a sample
counted immediately. Decay correction can be used
for samples counted up to 10 days after sampling,
though analytical precision will decline as the sample
gets weaker and weaker.
Ca = Rn / (Va + ∝ Vw) or
Rn = Ca ⋅ (Va + ∝ Vw)
Cw = Ca ⋅ (Va + ∝ Vw) / (Vw - Vh /∝)
With a standard RAD7, a laboratory drying unit and
typical room temperature, such that ∝ = 0.25, this
would reduce to:
Cw = Ca ⋅ (1.29 + 0.625) / (2.5 - 0.06)
or
Cw = Ca ⋅ 0.785
The decay correction is a simple exponential function
with a time constant of 132.4 hours. (The mean life
of a radon-222 atom is 132.4 hours, which is the half-
17
© 2015, DURRIDGE Company Inc.
3 CALCULATING RESULTS
life of 3.825 days multiplied by 24 hours per day
divided by the natural logarithm of 2.) The decay
correction factor (DCF) is given by the formula
(T/132.4)
DCF = e
in which T is the decay time in hours.
You will notice that decay times of under 3 hours
require very small corrections, and you can ordinarily
neglect the decay correction for samples that are
collected and then quickly brought back to the lab for
analysis.
To correct your result back to the sampling time,
multiply it by the decay correction factor (DCF) from
the chart, Fig. 9 on the next page.
3.3 Automatic Calculation with
CAPTURE Software
Durridge’s CAPTURE software for Windows and
Mac OS X makes it easy to obtain Big Bottle RAD
H2O radon data and automatically calculate radon in
water concentrations. CAPTURE’s Graph
Parameters window is used to specify details such as
the size of the water sample and whether a DRYSTIK
device was present during the aeration process, as
shown in Fig. 8, below. The program will then
automatically convert each downloaded RAD7 radon
reading into a radon in water data point, using the
procedure described in Section 3.2, above.
This functionality is described in depth in the
Opening Data From Disk section of the CAPTURE
user’s manual. The CAPTURE manual is available
in HTML and PDF formats at the Durridge website,
http://www.durridge.com/capture/.
Fig. 8 The CAPTURE Big Bottle RAD H2O configuration interface
18
© 2015, DURRIDGE Company Inc.
3 CALCULATING RESULTS
Fig. 9 Decay Correction Factors
19
© 2015, DURRIDGE Company Inc.
4 ACCURACY AND QUALITY CONTROL
4 ACCURACY AND QUALITY CONTROL
4.1 Calibration of System
The RAD H2O method relies on a fixed-volume
closed-loop extraction of radon from water to air.
Since the volumes are constant and the physical
properties of radon are constant, we do not anticipate
a need to routinely adjust the conversion coefficient.
The only factors we anticipate will require
"calibration checks" are sampling and laboratory
technique, and the RAD7.
In sample handling you can lose a significant fraction
of the radon if you do not follow consistent
procedures. For this reason we recommend that you
regularly review your method, and compare your
results to those of other methods in side-by-side
comparisons.
One way to check the accuracy of your analysis
technique is to take side-by-side samples of water
you know to be between 100 pCi/L and 10,000 pCi/
L (say, 4,000 Bq/m3 and 400,000 Bq/m3), one
sample in the BIG BOTTLE and the other in a
standard 250ml RAD H2O vial analyze one with the
BIG BOTTLE RAD H2O system and the other with
the standard RAD H2O system.
Another way would be to monitor a water supply
continuously with a RAD AQUA and take a sample
of that water in a BIG BOTTLE for analysis. Please
note that the RAD AQUA reading, because of its
delayed response, gives the radon concentration in
the water 30 minutes or more before the reading is
given. Thus when a sample for the BIG BOTTLE is
taken, the user should wait at least 30 minutes and
perhaps an hour before taking the latest RAD AQUA
reading. This precaution is only necessary if the
radon concentration in the water is changing.
As part of your quality assurance plan, you should
regularly check the RAD7 for its ability to determine
radon in air, and periodically send the RAD7 in for a
check-up and recalibration. Government agencies
usually recommend or require annual or bi-annual
recalibration of radiation measurement instruments.
You can find more information about calibration in
the RAD7 Operation Manual.
20
DURRIDGE recommends against the use of
radium-226 solutions in any RAD H2O or BIG
BOTTLE RAD H2O system due to the risk of
permanent contamination.
4.2 Accuracy and Precision
A number of factors affect the accuracy and precision
of a radon in water measurement. Most critical
among these factors is the sampling technique, which
was discussed in greater detail in a previous section.
Other factors include the sample concentration,
sample size, counting time, temperature, and
background effects.
4.2.1 Sampling Technique
Similar samples may vary by as much as 20% due to
uneven aeration, losses during sample acquisition
and transfer and/or variation of the water being
sampled, between earlier and later samples. By
paying very careful attention to detail it may be
possible to get the variation down to under 5%.
4.2.2 Sample Concentration
You can usually determine high concentrations with
a better precision than low concentrations (when the
precision is expressed in terms of percent
uncertainty). This is because a higher concentration
gives a greater number of counts per minute, yielding
more favorable counting statistics. If the
concentration is too high, however, you may exceed
the upper limit of the RAD7's linear range.
4.2.3 Sample Size
A larger sample size gives a greater number of counts
per minute, and so improves sensitivity and precision
at low radon concentrations. But the larger sample
size reduces the method's range, and increases
temperature effects. Thus the BIG BOTTLE RAD
H2O accessory is intended primarily for measuring
low radon concentrations, such as may be
encountered in open water or some domestic water
supplies.
© 2015, DURRIDGE Company Inc.
4 ACCURACY AND QUALITY CONTROL
4.2.4 Counting Time
Longer counting times improve sensitivity and
precision by accumulating a greater total number of
counts, which gives more favorable counting
statistics. With the BIG BOTTLE RAD H2O system,
the counting time is virtually unlimited. However,
factors such as the ambient air radon concentration
and other uncertainties limit the value of counting
times beyond about two hours.
4.2.5 Temperature
The effect of temperature on the calculation of radon
concentrations in 40ml vials is negligible, and for
250ml vials it only becomes significant at
temperatures outside 15 - 25 deg C (60 - 77 deg F).
For the BIG BOTTLE RAD H2O however it is
necessary to know the temperature of the water in the
bottle at the time of aeration to within +/- 1 deg C (2
deg F). The temperature logger supplied with the
equipment is designed to provide this information by
measuring the temperature of the glass about halfway
up the bottle.
The equilibrium ratio, ∝ of radon concentration in
water to air is given by:
(-0.0502 • T)
∝ = 0.105 + 0.405 ⋅ e
Where T ℃ is the temperature of the air-water
interface (Weigel, 1978).
At typical room temperature ∝ is about 0.25, making
the air-equivalent volume of the water about 0.6L.
4.2.6 Relative Humidity
Provided the desiccant does not get hydrated during
the measurement, or if an Active DRYSTIK is used,
the relative humidity in the RAD7 may be kept low
during the entire one or more hours of the
measurement. In any case, CAPTURE may always
be used to correct the readings, automatically, for
high humidity.
4.2.7 Background Effects
By careful attention to details, one can reduce the
background in any RAD H2O system to insignificant
levels. We previously discussed how to control the
background due to purge air radon content and
residual radon and its progeny. The uncontrollable,
or "intrinsic", background of the RAD7 is low
21
enough to ignore in all but the most demanding
cases. The intrinsic background of the RAD7 is less
than 1 count per hour, corresponding to a 40 mL
water sample concentration of less than 2 pCi/L (0.3
pCi/L for the 250 mL sample and eight times lower
still in the BIG BOTTLE RAD H2O system). In
principle, you can achieve a background this low if
you completely eliminate all radon and progeny from
the system before aerating the water, but that will
require a fair amount of effort and patience. A more
realistic background to shoot for, using the BIG
BOTTLE RAD H2O system, would be between 0.2
and 0.5 pCi/L (between 8 and 20 Bq/m3) in the
water. If you know the background well enough, you
can subtract it from the reading and have reasonable
confidence in the result.
4.3 Comparison of RAD H2O and BIG
BOTTLE with Other Methods
Fig. 10 is a table of typical numbers to give a basis
for comparison. Some laboratories may be able to
get better results with the RAD H2O than this table
indicates, while others may not. The precision
figures include counting statistics only, with no
adjustment for sampling variation or decay of the
sample.
Note that standard laboratory analysis often entails a
long delay between sampling and analysis, which can
significantly increase the error and raise the detection
limit (DL) and the lower limit of detection (LLD).
Also note that the background figure used to
calculate the RAD H2O and BIG BOTTLE RAD
H2O precision, DL, and LLD is conservatively
estimated to reflect typical field usage. The most
demanding and patient operator should be able to
reduce background to less than 0.02 cpm (rather than
the 0.10 cpm given in the table), which will allow for
DL's and LLD's lower than those listed.
4.4 Quality Assurance
A proper quality assurance plan should follow the
guidelines set by the USEPA as described in
[Goldin]. Compliance with certification programs
certainly require an approved quality assurance plan.
The elements of a quality assurance plan include
blank samples, duplicate samples, and spiked
samples. Often, the plan provides for blind samples
to be measured in an inter-comparison program. If a
quality control measurement deviates beyond the
© 2015, DURRIDGE Company Inc.
4 ACCURACY AND QUALITY CONTROL
acceptable range, the operator must cease to make
measurements until the cause of the deviation has
been discovered and corrected. Therefore, the
quality assurance plan should specify the range of
Method
Sample Size (ml)
Sensitivity (cpm/pCi/L)
Background (cpm)
acceptable measurement deviations, often in the form
of a "control chart". The operator should maintain
complete records of the quality control
measurements and their deviations.
RAD H2O 40
RAD H2O 250
Big Bottle RAD H2O
Liquid Scintillation
Lucas Cell
40
0.008
0.1*
250
0.05
0.1*
2500
0.3
0.1*
10
0.09
15
10
0.05
0.25
32
35
2-sigma uncertainty at 300 pCi/L (in pCi/L)
20-minute count
88
35
60-minute count
51
20
2.5
19
20
120-minute count
36
14
1.8
14
14
24
20
2-sigma uncertainty at 100 pCi/L (in pCi/L)
20-minute count
53
20
60-minute count
31
12
1.5
14
12
120-minute count
22
8.5
1.1
10
8.5
DL (C=2*(1+sqr(1+2*B)) in pCi/L) (NPDWR 40-CFR-41.25)
20-minute count
40*
6*
28
9
60-minute count
19*
3*
0.4*
16
4
300-minute count
7*
1*
0.18*
7
2
LLD (C=4*(1+sqrB)) in pCi/L (Altshuler)
20-minute count
60*
10*
41
13
60-minute count
29*
5*
0.6*
23
6
300-minute count
11*
2*
0.25*
10
3
Fig. 10 Method Comparison
* Note that the RAD7 background is conservatively estimated to be 0.1 cpm to arrive at these figures. With care
an operator should be able to reduce the background to less than 0.02 cpm, which would significantly lower the
RAD7 DL and LLD figures from those displayed.
22
© 2015, DURRIDGE Company Inc.
5 CARE, MAINTENANCE and TROUBLE SHOOTING
5 CARE, MAINTENANCE and TROUBLE SHOOTING
5.1 Warning on Pump Direction
The RAD H2O system cannot tolerate the reversal of
the air connections at the aerator head or the RAD7.
A check valve should be used at all times to prevent
the possibility of sucking water into the RAD7,
should a connector be accidentally reversed. If a
reversed connection occurs, the check valve prevents
the water from rising past the aerator head by
blocking its path. Do not allow the RAD7 to
continue pumping against a blocked check valve, as
this may cause damage to the pump or to the RAD7's
internal seals.
5.2 Warning on Tipping the Aeration
Unit
Never operate the BIG BOTTLE RAD H2O and
aeration unit in any position other than upright! If it
tips to any direction it may allow water to pass
through the outlet tube toward the RAD7. If liquid
water reaches the RAD7, it could permanently
damage critical internal parts, resulting in an
expensive repair bill.
If water ever enters the RAD7, or if the RAD7 ever
goes swimming in the water, it will probably cease to
operate and immediate steps should be taken to
minimize the impact on the instrument.
Keep the RAD7 upright. This will prevent water
from touching the detector, which is close to the face
plate at the top of the dome. Put a piece of tubing on
the RAD7 outlet with the other end in a sink. Use
the RAD7 pump if it still works or, otherwise, an
external pump into the inlet, to blow air through the
instrument. When water ceases to be blown out of
the outlet, put desiccant upstream of the RAD7 to dry
out the air path. When the air path is fully dry (after
dry air has been blown through it for approximately
one hour), remove the face plate from the case,
empty the water out of the case and blow dry the
case and the RAD7 electronics.
Once there is no visible water in or on the
instrument, it can be put in an oven at 50℃ for a few
hours to dry out completely. Additionally, desiccated
air can be passed through the air path until the air
leaving the RAD7 drops below 10% RH. After this
treatment further corrosion will be prevented, and the
RAD7 will boot once more and you can use the
23
internal RH sensor to measure how dry the air path
is. At this point the instrument should be returned to
DURRIDGE for service.
Use the large retort stand to support the desiccant, or
at least the bubble trap, in a vertical position, with
the 2.5L jug and aerator directly beneath it.
5.3 Stones Maintenance
After performing many radon in water
measurements, the stones may begin to show stains
or even begin clogging due to the buildup of mineral
deposits. If the mineral buildup is light and low in
radium content, we see no reason for concern.
Heavy deposits may be removed from the stones by
soaking it in an acid solution, followed by a thorough
rinse with clean water. Replacement stones are
readily available at any aquarium shop.
5.4 High Humidity
With cycles longer than 5 minutes, the pump
normally stops after the first five minutes. This is
insufficient to aerate the sample thoroughly. The
solution, after the measurement has started, is to set
the pump to ON [Setup]→[Pump] →[On]
[Enter] and leave it running for the entire first
cycle. After that, the bypass clip can be opened,
allowing the air flow to bypass the water, and the
pump set to AUTO, which would pump for five
minutes at the start of every cycle. This will keep the
humidity low throughout the measurement.
Without desiccant and without an Active DRYSTIK,
the pump will run continuously on AUTO and can be
turned off after 15 minutes of aerating the sample by
setting the pump to OFF. The data should be
downloaded with CAPTURE and displayed with the
“show corrected radon” button checked.
With an Active DRYSTIK, the RAD7 pump should
be set to OFF and the DRYSTIK pump used to
circulate air round the loop. After 45 minutes of
pumping and aerating the sample, the bypass clip can
be opened and the duty cycle of the DRYSTIK
changed to, say, 10%. The relative humidity in the
RAD7 may not be as low as with desiccant and it
may still be appropriate to click the “show corrected
radon” in CAPTURE.
© 2015, DURRIDGE Company Inc.
5 CARE, MAINTENANCE and TROUBLE SHOOTING
5.5 Foaming
While clean water causes no problem, some natural
waters contain foaming agents that will cause
bubbles to rise up out of the aerator. The bubble trap
supplied with the BIG BOTTLE RAD H2O is
designed to limit the upward migration of the foam.
This arrangement makes it difficult for bubbles to
rise up as far as the desiccant and reduces the
concern about foaming.
5.6 Glass Jug Care
As mentioned above, it is essential to leave a small
air pocket in the jug when it is sealed to prevent
24
smashing the glass when the water inside heats up.
CAPTURE provides automatic compensation for this
head space in the calculation of radon in the water.
To protect the jug from accidental damage when
carrying it around, protective shoulder bags are
available. Please see the Durridge web site
(www.durridge.com) for details.
5.7 Technical Support
It is not expected that the BIG BOTTLE RAD H2O
system will need maintenance or service in normal
use. For technical support please contact
[email protected], or call Durridge Company at
(978) 667-9556.
© 2015, DURRIDGE Company Inc.
6 BACKGROUND TO RADON-IN-WATER MEASUREMENTS
6 BACKGROUND TO RADON-IN-WATER MEASUREMENT
6.1 About Radon-in-Water
Radon originates from the radioactive decay of
naturally occurring uranium and radium deposits.
These elements can be found, in trace amounts, in
almost all soils and rocks. Being a gas, radon can
escape from mineral surfaces and dissolve in ground
water, which can carry it away from its point of
origin. We rarely find radon in high concentrations in
surface waters, due to its rapid dispersal into the
atmosphere.
High concentrations of groundwater radon prevail in
parts of New England, New Jersey, Maryland,
Virginia, and the mountainous western states of the
U.S. Typical groundwater sources average between
200 and 600 pCi/L of radon. Roughly 10 percent of
public drinking water supplies have concentrations of
over 1,000 pCi/L, and around 1 percent exceed
10,000 pCi/L. Smaller water systems appear to be
disproportionally affected by high radon. [Milvy,
EPA]
Radon was first noticed in water supplies by J.J.
Thomson, a pioneer in the science of radioactivity, in
the first decade of the 1900s. [Hess, Frame] At first,
scientists and doctors believed radioactivity to have
benign, even curative, effects on the human body.
Early research linked high radon concentrations to
natural hot springs long thought to have miraculous
powers. But eventually, science came to understand
the dangers of radiation exposure, after a number of
serious accidents and fatalities. [Caulfield]
In the 1950s airborne radon decay products emerged
as the probable cause of high incidences of lung
cancer among underground mine workers. Study of
environmental radioactivity revealed unusually high
groundwater radon concentrations in the vicinity of
Raymond, Maine. [Bell] In the 1960s, scientists
began to investigate the effect of ingested and inhaled
radon gas, observing the uptake of radon by digestive
organs and its dispersal through the bloodstream.
[Crawford-Brown] By the 1970s, radon was widely
recognized as a major component of our natural
radiation exposure. By the late 1970s, Maine had
initiated a program to attempt to reduce public
exposure to waterborne radon, having discovered
cases in which groundwater concentration exceeded 1
million pCi/L. [Hess]
Federal action on the problem of radon in drinking
water picked up in the 1980s with a nationwide
25
program to survey drinking water supplies for
radioactivity and to assess the risk to public health.
Congress directed the Environmental Protection
Agency (EPA) to take action on radioactivity in
drinking water, and in 1991 the EPA officially
proposed a Maximum Contaminant Level (MCL) for
radon in public drinking water of 300 pCi/L. This
MCL may one day become binding on public water
supplies. [Federal Register, EPA]
6.2 Health Risks Due to Waterborne
Radon
Waterborne radon leads to health risk by two
pathways: inhalation of radon and its decay products
following the release of radon gas from water into
household air, and the direct ingestion of radon in
drinking water.
The risk of lung cancer due to inhaled radon decay
products has been well documented through the study
of underground mine workers. The cancer risk due to
ingestion, primarily cancer of the stomach and
digestive organs, has been estimated from studies of
the movement of radon through the gastrointestinal
tract and bloodstream. Radon has not been linked to
any disease other than cancer. The cancer risk from
the inhalation pathway probably far exceeds that
from the ingestion pathway. [Crawford-Brown,
Federal Register]
In a typical house, with typical water usage patterns,
a waterborne radon concentration of 10,000 pCi/L
will yield an average increase to indoor air
concentrations of about 1 pCi/L. The 10,000:1 ratio,
while not to be considered a hard rule, has been
verified through theoretical models and empirical
evidence. [Hess] In a house with a high radon in
water content, air radon concentrations tend to rise
dramatically with water usage, especially in the
vicinity of the water-using appliance, but decline
steadily after the water usage tails off. [Henschel]
In most houses, waterborne radon is a secondary
source of indoor radon, far exceeded by soil gas
infiltration. It is an exception, though not a rare one,
that waterborne radon is the major contributor to
elevated radon in air. A homeowner who has
discovered elevated air concentrations, and whose
house uses private well water, should test the water
for radon content to assess the water's contribution to
the airborne radon. This test ought to be done before
© 2015, DURRIDGE Company Inc.
6 BACKGROUND TO RADON-IN-WATER MEASUREMENTS
any attempt to mitigate soil gas infiltration,
particularly if other wells in the area have been found
to have radon. [Henschel]
6.3 Physical Properties of Waterborne
Radon
Radon gas is mildly soluble in water. But, being a
gas, it is volatile. It tends to leave the water upon
contact with air. This is known as aeration.
The rate of radon transfer from water to air increases
with temperature, agitation, mixing, and surface area.
In household water usage, showers, baths,
dishwashers, laundries, and toilets all provide
adequate aeration to release a high percentage of the
water's radon content into household air. [Prichard]
In principle, the radon will continue to be released
from water as the aeration process continues, until a
state of equilibrium develops. According to Henry's
Law of dilute solutions, equilibrium will occur when
the water concentration and air concentration reach a
fixed ratio for a certain temperature. This ratio,
derivable from the Henry's Law constant for radon
dissolved in water, is known as the distribution
coefficient or partition coefficient.
For radon in water at 20 degrees C (68 F) the
distribution coefficient is about 0.25, so radon will
continue to release from the water until the water
concentration drops to about 25 percent of the air
concentration. Remember that as the radon leaves
the water into the air it raises the air concentration
and lowers the water concentration. At lower
temperatures the distribution coefficient increases,
rising to 0.51 at 0 degrees C (32 F). At higher
temperatures the distribution coefficient decreases,
dropping to about 0.11 at 100 degrees C (212 F). An
empirical expression for the distribution coefficient
of radon in water as a function of temperature can be
found in [Weigel].
6.4 Radon as a Tracer for
Groundwater movement
Soil and rock typically contain significant
concentrations of uranium and radium. Radon is
continually being created in the ground so that
groundwater often has high radon content. By
contrast, open water contains very little dissolved
radium. That, together with the proximity of the
water surface, means that the background
26
concentration of radon in sea and lake water far from
land is very low.
Radon, then, with its 4-day half life, is an almost
perfect tracer for measuring and monitoring the
movement of ground water into lake and sea water
along the shore [Lane-Smith, Burnett].
In open water, fast-response continuous radon
measurement at high sensitivity is provided by the
RAD AQUA [www.durridge.com]). For ground
water it is usually more convenient to use the RAD
H2O, but this is not sensitive enough for open water.
The gap between the RAD AQUA and the standard
RAD H2O is filled by the BIG BOTTLE RAD H2O.
This has one tenth the sensitivity of the RAD AQUA
and is eight times more sensitive than a standard
RAD H2O.
6.5 Standard Methods for Radon in
Water Analysis
Several methods have been developed to measure
radon in water. Three of these are Gamma
Spectroscopy (GS), Lucas Cell (LC) and Liquid
Scintillation (LS).
Gamma spectroscopy seeks to detect the gamma rays
given off by radon's decay products from a closed
container of radon bearing water. While simple in
concept, this method lacks the sensitivity to detect
radon at the lower levels now considered important.
The Lucas Cell method has been in use for decades
for laboratory analysis of radon-222 and radium-226
(via radon emanation). The LC method tends to be
somewhat labor intensive, using a complicated
system of glassware and a vacuum pump to evacuate
a Lucas (scintillation) cell, then bubble gas through
the water sample until the cell fills. The cell is then
counted by the usual technique. In the hands of a
skilled technician this method can produce accurate,
repeatable measurements at fairly low concentrations.
[Whittaker, Krieger (Method 903.1)]
The Liquid Scintillation method dates to the 1970s.
A liquid scintillation cocktail is added to the sample
in a 25 mL glass LS vial. The cocktail draws the
radon out of the water, so that when it decays the
alpha particles scintillate the cocktail. The method
uses standard LS counters, which are highly
automated and can count several hundred samples in
sequence without intervention. The EPA has
determined that the LS method is as accurate and
© 2015, DURRIDGE Company Inc.
6 BACKGROUND TO RADON-IN-WATER MEASUREMENTS
sensitive as the LC method, but less labor intensive,
and less expensive. [Prichard, Whittaker, Hahn
(Method 913.0), Lowry, Vitz, Kinner, Hess]
In comparison with the above, the RAD H2O offers a
method as accurate as LS but faster to the first
reading, portable, even less labour intensive and less
expensive. It also eliminates the need for noxious
chemicals. The BIG BOTTLE variant of the RAD
H2O has the same advantages as the standard RAD
H2O together with an 8-fold increase in sensitivity,
giving it a lower limit of detection that is an order of
magnitude better than liquid scintillation.
6.6 Mitigation Strategies
Two main strategies have emerged for the removal of
radon from water. Both of these are applicable to
point-of-entry (POE) water treatment in residences
and small public water supplies.
Granular Activated Carbon (GAC) attempts to filter
the water by adsorbing radon on a charcoal bed that
holds onto the radon until the radon decays. GAC
systems can be effective and relatively inexpensive
for residential use, but can create new problems. As
the radon and its progeny decay in the GAC column,
27
they give off gamma radiation. The gamma radiation
may be a health concern to residents when the
influent radon concentration is high, the GAC
column is poorly shielded for high energy radiation,
and the residents are likely to spend significant
periods of time in the radiation field. Over time, a
long lived decay product, lead-210, builds up in the
column, which may pose disposal problems in
systems with moderate to high radon concentrations
in the influent. For that reason GAC is most often
recommended for influent concentrations of up to
around 5,000 pCi/L. GAC maintenance is simple and
inexpensive, and the GAC bed has an expected
service life of 5 to 10 years. [Henschel, Lowry,
Rydell]
Aeration brings water into contact with a stream of
low radon air, which strips the radon from the water,
then exhausts the radon bearing air to the atmosphere.
Aeration systems offer effective removal of radon
without the buildup of gamma radiation or waste
material, but tend to be substantially more expensive
than GAC to install and maintain in a residential
setting. Aeration can be used over the entire range of
influent concentrations, though very high influent
concentration may require a multiple stage system to
reduce the effluent concentration to acceptable levels.
[Henschel, Lowry, NEEP]
© 2015, DURRIDGE Company Inc.
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© 2015, DURRIDGE Company Inc.